"Twilight" actor
Robert Pattinson was reportedly stunned by the news that his co-star and girlfriend of four years,
Kristen Stewart, admitted she had been
caught kissing her married
"Snow White and the Huntsman" director,
Rupert Sanders.
People magazine reports Pattinson, 26, has even moved out of the Los Angeles home he shares with Stewart.

According to the
New York Post, witnesses saw the couple arguing backstage at the Teen Choice Awards on Sunday (July 22), following 22-year-old Stewart's "marathon makeout session" (
as described by Us Weekly) with Sanders, 41, on July 17.

"Rob and Kristen were having a long and intense conversation backstage, and he looked miserable, heartbroken and humiliated. She was pleading," a source tells the paper.

On Wednesday, Stewart issued the following statement, "I'm deeply sorry for the hurt and embarrassment I've caused to those close to me and everyone this has affected. This momentary indiscretion has jeopardized the most important thing in my life, the person I love and respect the most, Rob. I love him, I love him, I'm so sorry."

Sanders put out his own statement saying, "I am utterly distraught about the pain I have caused my family. My beautiful wife and heavenly children are all I have in this world. I love them with all my heart. I am praying we can get through this together."

And in an unfortunately timed interview with YOU magazine just two weeks before news of the scandal broke, Sanders' wife,
Liberty Ross, talked about the isolation she feels in her marriage.

"I romanticized domesticity for a while, and loved having a shopping list of groceries stuck to the fridge for the first time," Ross tells YOU magazine (
excerpted by the Daily Mail). "But moving here and starting a life all over again was a lot harder than I had anticipated. I just didn't think it would be as isolating and daunting as it turned out to be ... I would never say out loud that I am raising my children alone, but a lot of the time it has felt like that. I went from the glamour of working with Karl Lagerfeld and John Galliano to living on an isolated hilltop, with my husband gone most of the time."

And the July 17 rendezvous between Stewart and Sanders apparently wasn't the first time the pair had gotten cozy. A source
tells People that while filming "SWATH," Sanders seemed infatuated with Stewart. A photo from May shows Stewart and Sanders having an intimate dinner with a small group in Berlin.

"She wasn't having an affair with Rupert. It was just a fleeting moment that shouldn't have happened," another source says. "She never meant to hurt anyone. She's a good person who just made a bad choice."